1. Field of the Invention
This invention relates to a memory system and a card controller. More particularly, this invention relates to a memory system which includes a nonvolatile semiconductor memory and a card controller for controlling the operation of the nonvolatile semiconductor memory.
2. Description of the Related Art
In a NAND flash memory, data is written into a plurality of memory cells simultaneously. A unit for simultaneous writing is called a page. With the recent trend of NAND flash memories toward higher capacity, the page size is getting larger.
However, a host unit does not necessarily provide access in pages. For example, in a write operation, the end position of write data may not be at the page boundary. Moreover, chances are high the host unit will provide write access again using consecutive addresses. The reason is that the host unit cannot send a large quantity of data at a time to a flash memory and therefore divides the data into pieces and writes them a plurality of times.
In a conventional NAND flash memory, the same page is generally allowed to be written into unless overwriting whereby the same column is written into twice is done. Therefore, in such a case, no problem occurs.
In recent years, however, memory cells have been miniaturized further and designed to have a multilevel configuration. With this trend, writing data into the same page a plurality of times becomes a problem in terms of reliability. Thus, NAND flash memories prevented from performing such an operation have been increasing.
Predicting access from the above-described host unit, a control system for a NAND flash memory, when consecutive write accesses end at a place which is not the end position of a page, writes the data on the page into another memory block. A memory block is a set of memory cells and serves as a unit for erasing data. In this connection, there is a known technique where a page size of data is stored in a buffer when a write end address has not been given in advance by the host unit or when the transfer of data from the host unit has been interrupted for some reason. Such a technique has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2006-092169. In the method, when the data size has finally failed to reach the page size, the data is written into another memory block.
However, the method has the problem of being unable to write data until the data size is eventually determined and therefore causing loss of time.